1. Field of the Invention
The present invention relates to a toggle-type mold clamping mechanism of an injection molding machine.
2. Description of Related Art
In a mold clamping mechanism of an injection molding machine, a movable die must be moved parallel to a stationary die to perform mold closing, mold clamping, and mold opening. Especially for the production of precise molded products, requirements of a clearance around and parallelism between die guide pins are strict, requiring highly accurate parallelism not only at the tune of mold clamping, but also at the time of mold opening. This indicates that the stationary and movable platens, to which the stationary and movable dies are attached respectively, must be moved parallel to each other to perform mold closing and opening actions.
In a toggle type mold clamping mechanism using a toggle mechanism therefor, links constituting the toggle mechanism include pivot joints each comprising a pin and a bushing of a slide bearing. Since the pin rotates relative to the bushing, a clearance must be provided between the pin and the bushing, and thus the clearance cannot be zero. The clearance degrades the parallelism between the stationary platen (stationary mold) and the movable platen (movable mold) at the time of mold closing or mold opening.
FIGS. 8a and 8b are explanatory views on the above point, in which L denotes a link; B, a bushing; and P, a pin. FIG. 8a shows a state at the time of mold clamping, in which a force of a driving source for mold clamping is transmitted to the pin P and to one end of the link L through the bushing B whereby the movable platen coupled to another end of the link is driven so that mold clamping is achieved. It is assumed here that the pin P is driven from the left to the right as shown by an arrow in FIG. 8a, and a mold clamping force a is transmitted to the link L. At this time, the clearance formed between the pin P and the bushing B is deviated to the left side in FIG. 8a where an amount of clearance is shown by δ. When the mold opening is performed from this state, the pin P is driven to the left. A mold opening force is shown by an arrow b in FIG. 8b. As shown in FIG. 8b, the pin P is moved by an amount corresponding to the clearance δ, independently of the link L. While the pin P is moved by the clearance δ, the link L can be moved freely without being restricted by the pin P.
As a result, even if the parallelism has been adjusted in the state shown in FIG. 8a in such a manner that the stationary platen (stationary die) and the movable platen (movable die) are in parallel to each other, there is a possibility that the parallelism is degraded during the mold opening shown in FIG. 8b due to the affection of the clearance δ. Conversely, even if the parallelism has been adjusted in the state shown in FIG. 8b, the parallelism can be degraded during the mold clamping shown in FIG. 8a. 
To obviate the above problem, there is known a method to constitute each of pivot joints of toggle links in the form of a rolling bearing structure to reduce a radial clearance (see JP 2584325B).
Also known is to provide fluid-pressure cylinders at four corners of the stationary platen for pressing the movable platen in the direction away from the stationary platen, and actuate these fluid-pressure cylinders at start of mold opening to move the movable platen in the mold opening direction, thereby maintaining the parallelism (see JP 7-88918A).
When precise molded products are to be injection-molded, it is an essential matter to maintain the parallelism between the stationary and movable platens at the time of mold closing, mold clamping, and mold opening. Thus, even a slight change in the parallelism caused by the presence of a clearance between the pin and the bushing in the toggle mechanism may be a problem. A method in which the bearing adopted in JP 2584325B is used in order to obviate the above problem results in drawbacks that ease of assembly and maintenance is impaired and noise at the time of mold opening/closing increases.
The method shown in JP 7-88918A to use fluid-pressure cylinders not only requires the fluid-pressure cylinders, but also requires a driving source and control means therefor and further driving energy. A further problem is the necessity of adjusting the fluid-pressure cylinders to meet a die thickness each time a die change is made.